1. Field of the Invention:
The present invention relates to a speed and a position control apparatus, and more particularly to a speed and a position control apparatus suitable for positioning a movable object such as a recording/reproducing element in a recording/reproducing apparatus.
2. Description of the Prior Art:
A control for positioning a movable object to a specific place has been carried out in various technical fields. For example, in the case where information signals are recorded into an optical disk or a magnetic disk or, to the contrary, the recorded information are reproduced from the optical disk or the magnetic disk, the recording/reproducing operation is normally carried by recording the information to a predetermined position of the disk and, in turn, the recorded information is searched for a reproduction. In such a case, it is necessary to accurately position a recording/reproducing element such as an optical head or a magnetic head to a target position within a short period of time.
FIG. 4 is a perspective view showing a recording/reproducing apparatus of an optical disk embodied as one example of a recording/reproducing apparatus. In FIG. 4, a reference symbol M denotes a motor causing a driving force for turning a disk. A reference numeral 1 denotes an optical disk, and a reference numeral 2 denotes a carriage. A reference numeral 5 denotes a lens provided on an optical head, and a reference numeral 6 denotes a magnetic path member for a linear motor. A reference numeral 7 denotes an actuating coil for a linear motor.
When information signal is recorded into the optical disk or the recorded information is reproduced from the optical disk 1, the motor M is actuated to rotate at a predetermined speed. In order to promptly position a condensing point of a recording light or a reproducing light focused by the lens 5 to a target track position on the optical disk 1, a speed control signal and a position control signal are successively supplied to the actuating coil 7 of the linear motor in response to both a target position signal given from a controller (not shown) and a position signal outputted from a position detecting device in accordance with a mutual distance of an optical scale (a linear scale) installed on a movable portion and a detector fixed to a non-movable portion.
Upon receipt of the speed control signal and/or the position control signal, the linear motor shifts the carriage 2 along guide rods so as to promptly and accurately position the condensing point of the recording/reproducing light focused by the lens 5 of the optical head to the target track position on the optical disk 1 in compliance with a seeking operation using a predetermined speed control mode, followed by a predetermined position control operation.
By the way, the speed control system in such a conventional apparatus was equipped with an encoder consisting of an optical scale and a detecting device for executing a speed control and/or a position control for a movable object. This encoder generates false sine-wave two-phase signals having a 90-degree phase difference with each other. These two-phase signals are differentiated and, in turn, added with each other. Then, a low-pass component of the added signal is used as a speed control signal.
FIG. 5 is a view illustrating a procedure for generating a speed signal in a conventional apparatus. Signals shown by (a), (b) of FIG. 5 show false sine-wave two-phase signals Sa, Sb which are generated in the encoder and have a 90-degree phase difference with each other, respectively. Signals shown by (c), (d) of FIG. 5 show signals Sa', Sb' which are absolute differentiated values of the false sine-wave two-phase signals Sa, Sb. Furthermore, a signal shown by (e) of FIG. 5 is a summation of two absolute differentiated signals Sa', Sb'. And, a signal shown by (f) of FIG. 5 is a speed signal Ss obtained by processing the summation signal of the two absolute differentiated signals Sa', Sb' through a low-pass filter.
However, thus obtained speed signal Ss has a disadvantage in that it has no information relating to a moving direction of the carriage.
Signals (a), (b) of FIG. 6 show a condition of the detecting signals Sa, Sb in the case where a shifting direction of a head is changed to an opposite direction. Signals Sac, Sbc shown by (c), (d) of FIG. 6 are obtained after comparing the detecting signals Sa, Sb with their mean values. A signal (e)-(1) shows a level when the signal Sbc is latched by the front edge of the signal Sac, and a signal (e)-(2) shows a level when the signal Sbc is latched by the rear edge of the signal Sac. Similarly, a signal (e)-(3) shows a level when the signal Sac is latched by the front edge of the signal Sbc, and a signal (e)-(4) shows a level when the signal Sac is latched by the rear edge of the signal Sbc.
In this case, if the level values of the signals (e)-(1) through (e)-(4) are changed in specific regions shown, for example, by U-1, U-2, U-3, and U-4, this shows that the shifting direction of the head is changed in a region V which is a common region of these U-1, U-2, U-3, and U-4 regions. A length of the region V corresponds to 1/4 of a period of the signal outputted from the encoder. In other words, a resolution of this conventional method for detecting a turning point of the moving direction is recognized as 1/4 of the period of the signal outputted from the encoder. Therefore, it was difficult to finely detect the turning point.
Furthermore, a position signal used for a position control in a conventional apparatus is only one of the above-described false sine-wave two-phase signals having a phase difference of approximately 90 degrees. FIG. 9 is a view illustrating a position detecting operation in a conventional apparatus. In FIG. 9, portions circled and indicated by arrows show intersections of a reference voltage V.sub.ref and a position control signal; i.e. a stable point for the positioning operation.
In the case where the position control is carried out by using only one of two-phase signals as shown in FIG. 9, the stable point for the positioning operation exists at every pitch of the encoder and the position signal (an error signal for a position control) generated in the encoder has a cyclic tendency. Therefore, the pull-in range of the position control is only a half pitch of the encoder signal.
Especially, it is recognized such a phenomenon that the shorter the cycle (displacement) becomes the smaller the holding torque becomes. Accordingly, if it is adopted for the position control of the carriage 2 in the recording/reproducing apparatus as shown in FIG. 4, positioning the carriage 2 to a target position after seeking operation becomes difficult as the pitch of the encoder becomes small.